The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process (U.S. Pat. No. 2,297,691) involves placing a uniform electrostatic charge on a photoconductive insulating layer known as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic toner material. The toner will normally be attracted to those areas of the photoreceptor which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image subsequently may be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment.
Toners and developer compositions including colored particles are well known. Some U.S. patents in this regard are U.S. Pat. Nos. 5,352,521, 4,778,742, 5,470,687, 5,500,321, 5,102,761, 4,645,727, 5,437,953, 5,296,325 and 5,200,290. The traditional compositions normally contain toner particles consisting of resin and colorants, wax or a polyolefin, charge control agents, flow agents and other additives. A typical toner formulation generally contains about 90-95 weight percent resin, about 2-10 weight percent colorant, 0-about 6 weight percent wax, 0-about 3 weight percent charge control agent, about 0.25-1 weight percent flow agent and 0-about 1 weight percent other additives. Major resins are styrene-acrylic copolymers, styrene-butadiene copolymers and polyesters. The colorants usually are selected from cyan dyes or pigments, magenta dyes or pigments, yellow dyes or pigments, and mixtures thereof.
One of the main advantages of selecting organic dyes instead of pigments for color toner compositions resides in the provisions of increased color fidelity as the dyes can be molecularly dispersed in the toner resins. To obtain a homogeneous dispersion, it is generally necessary to build into these molecules certain substituents for enhancing their compatibility with the toner resin. Unless the dye molecules are substantially fully compatible with the toner resins, they have a tendency to aggregate with time, especially when subjected to heat, pressure and humidity thereby resulting in a loss of color fidelity. Additionally, the low molecular weight of the dye molecules causes a high lability or mobility of the dye molecules in the toner resin resulting in undesirable bleeding of the dyes.
An attempt for improvement is to incorporate a dye into preformed resin particles by dispersing the particles in a dye solution and diffusing the dye into the central portion of each resin particle. For example, U.S. Pat. No. 5,565,298 discloses a method of producing toner particles comprising of a copolymer of styrene and n-butylmethacrylate formed by a suspension polymerization method and dyed by dispersing in a bath comprising of a dye and methanol as solvent. However, the method has several deficiencies that make it unsuitable for producing high-resolution toner particles. The dyeing has to be carried out below the glass transition temperature of the resin and it therefore takes a long dyeing time. Particles also tend to coagulate in the course of dyeing resulting in a large average particle size and a broad size distribution. Incorporating a sufficient amount of dyes for vivid color image is difficult due to a limited solubility of dyes in polymer resins. Dyes tend to migrate out of the particle during storage and evaporate during the fixing stage of electrophotography process, severely interfering with operation of electrophotography equipment.
There is continuing interest in the development of new and improved methods of producing toners for application in high-resolution color electrophotography. Accordingly, an object of the present invention is to provide a method of producing high-resolution color toner which has a superior combination of properties for electrophotographic imaging systems by dispersing resin particles and a dye in a bath and effecting the dye molecules to be absorbed in the central portion of each resin particle while substantially maintaining the size and size distribution of the resin particles.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.
There is provided in accordance with the present invention a process of preparing a toner for developing latent electrostatic images comprising: dispersing a particulate polymer resin with functional sites suitable for interacting with a functionalized dye in a liquid organic medium; the polymer being substantially insoluble in the organic medium; providing a functionalized dye to the organic medium wherein the functionalized dye has functional sites adapted for interacting with the functional sites on the particulate polymer resin; maintaining the organic medium containing the particulate resin at an elevated temperature for a time sufficient to dye the resin and separating the organic medium from the particulate polymer resin. The functionalized dye is thus applied to the resin particles and the particle size of the particulate polymer resin is substantially unchanged during the dyeing process recited above.
The particulate polymer resin is most preferably a polyester resin. The polyester resin may have functional sites suitable for interacting with a functionalized dye selected from the group consisting of: hydroxyl moieties; alkoxyl moieties; sulfonic or derivatized sulfonic moieties; sulfonic or derivatized sulfonic moieties; carboxyl or derivatized carboxyl moieties; phosphonic or derivatized phosphonic moieties; phosphinic or derivatized phosphinic moieties; thiol moieties, amine moieties; alkyl amine moieties; quaternized amine moieties; and mixtures thereof. In typical embodiments the particulate polymer resin has a volume average particle size of from about 1 to about 15 microns. Generally at least about 80 weight percent of the particles of the particular polymer resin are within from about 0.5 to about 1.5 times the volume average particle size of the particulate polymer resin. In other embodiments the particulate polymer resin has a volume average particle size from about 2 to about 10 microns and sometimes from about 2 to about 4 microns while an average particle size of from about 5 to about 8 microns is preferred in some embodiments.
In some cases the polyester resin is prepared by way of dispersion polymerization.
Any suitable dye may be used in the practice of the present invention so long as it can be bound to the particulate polymer resin. Preferred dyes include basic dyes, acid dyes, or reactive dyes. The weight ratio to dye to particulate polymer resin is generally from about 1:100 to about 10:100 or from about 1 to about 10 percent by weight.
The solubility parameter value of the organic medium is smaller than the solubility parameter value of the particulate polymer resin by at least about 1. More preferably the solubility parameter of the organic medium is smaller than the solubility parameter value of the particulate polymer resin by at least about 2. Particularly preferred are paraffin containing organic media.
A dyeing aid, typically a surfactant, is preferably included in the inventive process. Most preferred are non-ionic surfactants as detailed further herein. Especially useful non-ionic surfactants include the residue of an ethylene oxide moiety or a propylene oxide moiety.
The surfactant may be present in an amount of from about 0.2 to about 2 times the amount of non-polar solvent present in the organic medium, that is from about 5 to about 200 percent by weight of the non-polar solvent, whereas from about 10 to about 50 percent is more typical with from about 20 to about 40 weight percent of surfactant being preferred.
It is likewise preferred to operate the inventive process at relatively high solids content wherein the polymer resin is present in an amount of from about 10 to about 70 volume percent of the combined volume of resin and organic medium during dying. From about 20 to about 40 volume percent resin is perhaps more typical in some embodiments.
The elevated temperature at which the process of the invention is carried out is generally greater than 20xc2x0 C. less than the glass transition temperature of the resin being dyed. For example, a resin having a glass transition temperature of 100xc2x0 is dyed at a temperature greater than about 80xc2x0 C. During the dyeing process the organic medium is maintained at an elevated temperature which is typically higher than the glass transition temperature of the particulate polymer resin so that the dye and the charge control agent can readily penetrate the resin. Particularly preferred in some embodiments is an elevated temperature of at least about 30xc2x0 C. higher than the glass transition temperature of the polymer resin. Typically the polymer is dyed for at least five minutes and in many embodiments between about 5 and about 60 minutes.
A charge control agent is preferably added during the step of dyeing the particulate resin so as to simplify processing.
There is provided in another aspect of the present invention a dispersion dyed color toner for developing latent electrostatic images. The inventive toner is prepared by a process including dispersing a particulate polymer resin provided with functional sites suitable for interacting with a functionalized dye in a liquid organic medium, the polymer being substantially insoluble in the organic medium; providing the functionalized dye to the organic medium, wherein the functionalized dye has functional sites adapted for interacting with the functional sites on the particulate polymer resin; maintaining the organic medium, containing the particulate polymer resin and the dye at an elevated temperature for a time sufficient to dye the resin; and separating the organic medium from the particulate polymer resin. The functionalized dye is thus applied to the resin particles and the particle size of the particulate polymer resin is substantially unchanged during the process of preparing the toner.
In most embodiments the color toner also includes a charge control agent present in an amount from about 0.1 weight percent to about 10 percent by weight of the toner. The toner may optionally include a flow improvement agent such as fumed silica.
There is provided in still yet another aspect of the present invention a developer composition comprising the dispersion dyed color toner of the present invention. The developer composition includes the toner and carrier particles selected from the group consisting of ferrite particles, steel powder, iron powder and the like having a surface active agent coated therein. Examples of the carrier composition are described in U.S. Pat. No. 5,693,444.
As the resins for preparing toner particles for thermal image fixing, the conventionally known resins such copolymers of styrene and acrylate and polyesters. Polyesters are preferred for color toner applications because of their superior compatibility with colorants and adhesion to various printing substrates.
Furthermore, the resins, suitable for the inventive process, are chemically modified to contain one or more reactive functionalities in about 1-10 mole percent amounts. The reactive functionalities are chosen as to be reactive toward suitable dyeing reagents either by a covalent bonding or by ionic complexing mechanism. Examples of the functional groups include, but are not limited to, the moieties hydroxyl, alkoxy, sulfonic or derivatized sulfonic, sulfinic or derivatized sulfinic, carboxyl or derivatized carboxyl, phosphonic or derivatized phosphonic, phosphinic or derivatized phosphinic, thiol, amine, alkylamine and quaternized amine and combinations thereof, e.g., xe2x80x94S03M, Oxe2x80x94COOM, xe2x80x94P(xe2x95x90O)(OM)2, xe2x80x94P(xe2x95x90O)R(OM), xe2x80x94OH, xe2x80x94OR, xe2x80x94NR1R2R3N, xe2x80x94NHR and xe2x80x94SH, where R, R1, R2 and R3 are alkyl groups, M is a metal group and N is an anion.
In the present invention, it is preferable to use small resin particles which have a volume average particle size (L) in the range 1-15 xcexcm. The terms xe2x80x9cvolume average particle sizexe2x80x9d is defined in, for example, Powder Technology Handbook, 2nd edition, by K. Gotoh et al, Marcell Dekker Publications (1997), pages 3-13. More specifically, it is preferable to use resin particles which include resin particles with a particle size distribution in the range of 0.5 xc3x97L to 1.5 xc3x97L in an amount of 80 wt. % or more of the entire weight of the resin particles. This is because the resin particles with such a narrow particle size distribution provide toner particles which are uniformly dyed, have uniform quantity of electric charge in each toner particle, and can provide high-quality copy images and for which charge control is easy in a development unit.
In the present invention, the particle size distribution is measured by a commercially available Coulter LS Particle Size Analyzer (made by Coulter Electronics Co., Ltd., St. Petersburg, Fla.).
The desired polyesters of suitable particle shape and size may be prepared from the above-noted components by a variety of techniques. In order to prepare resin particles with the above-mentioned mean particle size and narrow particle size distribution, a dispersion polymerization method, in particular, the dispersion Ad polymerization method disclosed in British Patent 1,373,531, is suitable. The disclosure of the ""531 patent is incorporated herein by reference. Generally in a typical dispersion process, polymerizable monomers, an initiator and a dispersion stabilizer are dispersed in a solvent which is immiscible with the monomers. Under a vigorous shearing action, the monomers are finely dispersed as small droplets in the solvent and the droplets are stabilized without coalescence by the presence of the stabilizer molecules on their surface. The dispersion is then heated to an initiation temperature and the polymerization proceeds in each droplet. After a specified polymerization period, the reaction mixture is cooled to ambient temperature and polymer particles are separated by filtration for further processing. In the process, the particle size is controlled by the amount of added stabilizer and the shearing. The molecular weight of the polymer is controlled by the initiator amount and/or the polymerization time.
Optionally, the resin particles may be prepared by a milling process commonly used in preparing conventional toners and described, for example, in U.S. Pat. No. 5,102,761. In that process, a polyester resin is mechanically crushed, milled into small particles and then classified to obtain particles with desired particle size and size distribution.
The advantage of these resin particles is that they can be directly dyed by appropriately reacting the functionalities on the polymer with appropriate coloring reagents. The coloring reagent is typically a dye which may be a basic dye, acid dye, reactive dye and combinations thereof. Basic dyes are cationic molecules which ionically bind to anionic sites. Acid dyes are anionic molecules which bind to cationic or basic sites, while reactive dyes are functional molecules which contain groups that covalently bind to sites such as, for example, xe2x80x94OH, xe2x80x94SH or xe2x80x94NRH in order to form respectively an ether, thioether or amine linkages.
The weight ratio of the dye to the resin to be dyed can be selected as desired, depending upon the desired color tone. However, generally it is preferable that the amount of the dye is in the range of 1 to 10 parts by weight to 100 parts by weight of the resin particles to be dyed.
It is preferable to employ a solvent in which the resin particles are not soluble. More specifically, it is preferable that the solubility parameter value of the solvents is smaller than that of the resin particles by 1.0 or more, more preferably 2.0 or more. For example, it is preferable to employ a non-polar organic solvent having a low solubility parameter value such as paraffins, paraffinic esters, paraffinic amides and paraffinic ethers in combination with the styrene-acrylic resin particles or the polyester resin particles. In contrast, when a highly polar solvent such as water, methanol, propanol, and acetone is employed as a solvent for the dyeing process, significant coalescence of the particles occurs.
Particularly preferred organic media for use in connection with the invention are paraffins. Examples of paraffins are normal and isoparaffins with 7 or more carbon atoms such as: octane, decane, dodecane, and isoparaffinic mixtures sold under the name xe2x80x9cIsopar(copyright)xe2x80x9d by Exxon Chemical Company, Houston, Tex. Grades and their carbon numbers are as follows: Isopar(copyright) C C7-8; Isopar(copyright) E, C8-9; Isopar(copyright) G C10-11; Isopar(copyright) H C11-12; Isopar(copyright) K C1-12; Isopar(copyright) L C11-13; Isopar(copyright) M C13-14; and Isopar(copyright) V C12-40. These Isopar(copyright) are manufactured by distillation and each designation refers to the take off positions of a distillation column. Also suitable for organic media to be utilized in the dyeing process of the present invention are mineral oils which are mixtures of paraffins. So also paraffinic esters such as dodecyl acetate may be employed; whereas paraffinic amides such as decylamine may also be employed.
A surfactant is used in conjunction with the aforementioned non-polar solvent in the dyeing operation of this invention. The surfactant performs two important functions for successful dyeing of the particles. First, it prevents coalescence of the resin particles during the dyeing reaction. In the inventive process, dyeing is carried out generally at a temperature higher than the glass transition temperature of resin. Thus, in the absence of the surfactant, the particles are in the molten state, tend to coalesce in an uncontrollable manner and produce dyed particles which are unsuitable as a high-resolution toner. Secondly, the functional dyes employed in the present invention are generally insoluble in non-polar solvents and a means of delivering the dye molecules to the resin particles does not exist in the absence of the surfactant. The surfactant, having polar sites in its molecular structure and thus some solubility of the dye, plays the important role of transporting dye molecules from the dye particles to the resin particles and thus enabling the dyeing without a substantial particle agglomeration even when the amount of the resin to the solvent is as high as 100 parts by weight to 100 parts by weight of the total liquid medium in dye bath. The surfactant may be anionic, cationic or non-ionic. It is preferable that the surfactant is non-ionic.
The weight ratio of the surfactant to the non-polar solvent can be selected as desired depending on the amount of the resin particle to be dyed and the required processing time. However, generally it is preferable that the amount of the surfactant is in the range of 5 to 200 parts by weight to 100 parts by weight of the non-polar solvent. From about 10 to about 40 percent by weight of surfactant is somewhat typical, based on the weight of solution. The amount of the total liquid medium in dye bath to the resin to be dyed can be selected as desired. However, generally it is preferable that the amount of the solvent is in the range of 50 to 1000 parts by weight to 100 parts by weight of the resin particles to be dyed.
Examples of useful classes of non-ionic surfactants include alkylphenol ethoxylates, aliphatic alcohol ethoxylates, fatty acid alkoxylates, fatty alcohol alkoxylates, block copolymers of ethylene oxide and propylene oxide, condensation products of ethylene oxide with the product resulting from the reaction of propylene oxide and ethylenediamine and condensation products of propylene oxide with product of the reaction of ethylene oxide and ethylenediamine. Particularly useful surfactants include the reaction product of a fatty acid or a fatty alcohol with ethylene oxide such as a polyethylene glycol diester of a fatty acid (PEG diols or PEG diesters). A particularly preferred surfactant for use in the connection with the present invention includes Genapol(copyright)-26-L-1 surfactant available from Clariant Corporation which has the chemical structure of C13H27xe2x80x94C6H4xe2x80x94(xe2x80x94CH2xe2x80x94CH2Oxe2x80x94)xe2x80x94CH2xe2x80x94CH2xe2x80x94OH.
In the present invention, the dyeing is carried out, for example, by dispersing an appropriate functional dye in the above-mentioned mixture of a non-polar solvent and a surfactant, then dispersing the resin particles in the bath and stirring the dispersion under the conditions that the temperature of the dispersion is kept at a temperature of about 30xc2x0 C. or higher than the glass transition temperature of the resin. The high temperature ensures the penetrating rate of the dye into the resin particles to be sufficiently high that dyed resin particles can be obtained in about 5 minutes to about 60 minutes. For agitating the dispersion of the dye and resin particles, a conventional stirrer such as a blade-type mixer or a magnetic stirrer can be employed.
In the above-mentioned processes, dyed slurry is obtained. Dyed resin particles can be obtained from the slurry by any conventional methods. For example, dyed resin particles are separated from the slurry by filtration. The non-solvent and the surfactant are entrained in the filter cake and they are washed with a hydrocarbon with a low boiling temperature such as n-pentane, n-hexane, iso-hexane and the like. It is important not to use a polar organic solvent such as methanol, propanol or isobutanol for the washing since the cake tends to agglomerate upon exposure to such a solvent. The washed particles are then dried at a temperature below the glass transition temperature of the resin, or under reduced pressure. The thus obtained toner particles have substantially the same particle size distribution as that of the original resin particles.
In the present invention, in order to improve the triboelectric charging characteristics of the toner, charge control agents (xe2x80x9cCCAxe2x80x9d) which are conventionally known in this field can be contained in the toner particles. Suitable charge control agents may be the negative-type or the positive-type. Several such CCAs are commercially available such as, for example, the Bontron(copyright) E-88 brand CCA (a negative charge control agent which is an aluminum compound, available from Orient Chemical Corporation, Springfield, N.J.) and the Bontron(copyright) P-53 brand CCA (a positive CCA, also available from Orient Chemical Corporation). Such processes as dry mixing, solvent coating, spray coating and like may be used.
In the inventive process, a CCA is dissolved in an organic solvent mixture, specially prepared to prevent agglomeration of the dyed resin particles during CCA application, and either the dyed resin particles are immersed in the CCA solution at an elevated temperature conducive for diffusing-in of the CCA into the central portion of the particles or the solution is sprayed onto the dyed particles. Subsequently, the organic solvent is removed by drying, whereby the CCA is caused to stay in the central portion of the toner particles or on the surface of the toner particles, respectively. It is preferable that the solvent mixture used for the CCA application is the same solvent mixture used in the aforementioned dyeing process.
As another method of incorporating the charge control agent in the toner particles, a mechanical deposition method can be employed, in which a CCA, preferably with a particle size of 1 xcexcm or less, is mechanically fixed to the surface of the toner particles by causing the CCA particles to collide with the toner particles with application of mechanical energy thereto, when necessary, under application of thermal energy, whereby the CCA is fixed to the surface of the toner particles to such a fixing degree that the CCA does not come off the toner particles while in use.
For this mechanical deposition method, for example, a mixing apparatus such as ball mill, V-blender, or Henshel Mixer, is employed for mixing the CCA and the toner particles. Mechanical energy is then applied to this mixture, for instance, by rotating the mixture with rotary blades which are rotated at high speed, or by causing the CCA particles to collide with the toner particles within a stream of air which flows at high speed, or by causing both particles to collide with a collision plate in such an air stream, whereby the CCA is firmly fixed to the surface of the toner particles.
As commercially available apparatus for the above purpose of applying such mechanical energy, for instance, an apparatus named Mechanofusion(copyright) (made by Hosokawa Micron Co., Ltd., Summit, N.J.), a crushing mill which is modified so as to reduce crushing air pressure as compared with that of an ordinary crushing mill.
In the present invention, it is preferable that the amount of the CCA is 0.1 to 10 parts by weight to 100 parts by weight of the dyed resin particles for appropriately controlling the triboelectric charging characteristics of the toner particles and image fixing performance, although the above ratio can be varied, depending upon the charge quantity required for the toner particles or a development means for use with the toner particles.
The CCA-containing particles may then be coated with a suitable flowability improvement agent. They generally help to enhance the flowability of the particles during their use as color toner. Suitable flow agents are materials such as finely-divided particles of hydrophobic silica, titanium oxide, zinc stearate, magnesium stearate and the like which may be applied by processes such as, for example, dry mixing, solvent mixing and the like. In a typical process, a hydrophobic fumed silica (previously treated with a surface activating reagent such as, for example, hexamethyldisilazane and available under the trade name Cab-O-Sil(copyright) T-530 from Cabot Corporation, Tuscola, Ill.) is mixed with the CCA-coated particles and blended well in a tumble mixer for about 10-60 minutes to obtain flow agent-coated toner particles.
In many color toner applications, the toner particles are used as a developer which typically contains the dyed particles as described above (containing the CCA and the flow agent) and a suitable carrier agent (such as, for example, ferrites, steel, iron powder and the like, optionally containing a surface treating coating agent thereon) are mixed together intimately to form the developer.
The features of the present invention will become apparent in the course of the following description of examples, which are given for illustration of the invention and are not intended to be limiting thereof.